High pressure gas atomization (HPGA) as described in the Ayers and Anderson U.S. Pat. No. 4,619,845 issued Oct. 28, 1986, has shown considerable promise as a method of making very fine metal and alloy powder having a rapidly solidified microstructure with attendant control of alloyant segregation, grain size, supersaturation, and particle size, shape and distribution. The '845 patent describes atomization parameters required to effectively use the kinetic energy of supersonic (Mach 3 to 4) gas jet streams to disintegrate a melt into ultrafine, spherical powder particles. In particular, high pressure gas atomization in accordance with that patent employs an atomizing nozzle having multiple discrete, circumferentially spaced gas discharge orifices arranged about a nozzle melt supply tube having a central melt discharge orifice and adjacent frusto-conical surface. High pressure gas (typically an inert gas, such as argon, for reactive metals) is supplied to the gas discharge orifices for discharge in flush, laminar manner over the frusto-conical surface. The pressure of the gas supplied to the gas discharge orifices is selected at a high enough level (e.g., about 1800 psig) to establish a subambient pressure region immediately adjacent the melt discharge orifice to create an aspiration effect that draws melt out thereof for atomization at an apex region on the adjacent frusto-conical surface. The gas jet streams atomize the melt and form a narrow, supersonic spray containing very fine melt droplets that solidify rapidly as powder particles that are collected for further processing.
In addition to gas pressure, certain geometries/dimensions of the atomizing nozzle have been found to be important in achieving satisfactory atomization of the melt in the HPGA regime of operation. For example, the apex angle of the gas discharge orifices relative to the apex angle of the frusto-conical surface of the nozzle melt tube as well as the extension of the frusto-conical surface axially beyond the gas discharge orifices have been found to be important parameters that must be properly controlled and selected. Moreover, the orientation of the gas discharge orifices relative to the frusto-conical surface of the melt supply tube has also been found to be important. In particular, for optimum atomization, the gas discharge orifices should be flush with the frusto-conical surface and not offset therefrom. Detailed descriptions of the HPGA technique and the important parameters involved are set forth in the aforementioned patent and in the Anderson et al technical article (#1) "Fluid Flow Effects In Gas Atomization Processing", International Symposium on the Physical Chemistry of Powder Metals Production and Processing, TMS, Warrendale, Pa. (1989) and the Figliola et al technical article (#2) "Flow Measurements In Gas Atomization Processes", Synthesis and Measurements in Gas Processing: Characterization and Diagnostics of Ceramic and Metal Particulate Processing, TMS, Warrendale, Pa., pp. 39-47 (1989).
The second technical article referred to above describes modifications to certain geometries/dimensions of an original atomizing nozzle (developed by the Massachusetts Institute of Technology and described in its technical literature as an ultrasonic gas atomization nozzle) resulting in enhancement of the aspirating effect produced at the melt discharge orifice without affecting the location of the minimum pressure region relative thereto. In particular, the diameter of the melt supply tube was slightly enlarged to locate the gas discharge orifices flush with the frusto-conical surface adjacent the melt discharge orifice. An annular gas manifold was adopted although the gas jet ring diameter remained the same. The annular gas manifold was modified to have the gas discharge orifices communicate directly therewith rather via an intermediate annular manifold passageway as present in the M.I.T. device. The annular manifold was supplied with high pressure gas via a cylindrical conduit extending perpendicular to the manifold axis.
In attempting to atomize certain rare earth-transition metal magnetic alloys (e.g., rare earth-iron-boron alloys) using a gas atomizing nozzle modified as described in the aforementioned second technical article, the particular alloy chemistries involved were observed to adversely affect the atomization performance of the nozzle in so far as the particle sizes produced were distributed over a rather wide particle size range than expected, resulting in a higher than expected average particle size. In particular, a majority of the particle sizes produced were greater than an optimum particle size range (e.g., 3 to 44 microns, preferably 5 to 40 microns) where optimum magnetic properties are exhibited by the as-atomized particles for the particular alloy compositions involved. As a result, there is a need to further improve the performance of the gas atomizing nozzle in producing rare earth-transition metal magnetic alloy powder (e.g., rare earth-iron-boron alloy powder) to enable powder production with improved distribution of particle sizes in the optimum finer particle size range for the alloy compositions involved.
An object of the present invention is to provide a high pressure gas atomizing nozzle and atomizing method characterized by improved atomization performance.
Another object on the present invention is to provide an improved, efficient high pressure gas atomizing nozzle and atomizing method capable of producing rapidly solidified powder particles, especially of rare earth-transition metal alloys, wherein the percentage (yield) of particles falling within a desired fine particle size range for optimum properties (e.g., magnetic properties for the rare earth-transition metal alloys) is substantially increased so as to thereby increase the yield of the atomizing process.
Still another object of the present invention is to provide an improved, efficient high pressure gas atomizing nozzle and atomizing method capable of producing fine, rapidly solidified powder particles at a lower gas pressure, thereby reducing the quantity and supply pressure of the gas required and the cost of producing the powder particles.